Fractionation control



Oct. 15, 1963 E. D. TOLIN FRACTIONATION CONTROL Filed July 29, 1960 Lw t 3| 33 201 I9 24 AT l TRANsDuCER ACCuMuLATOR LC -1 V 22 23 |o\ v A J 2|-l Q6All -2 2 g l OVERHEAD l2 I PRODUCT I r 1 g AIP i i TRANsDuCER i l 3o Il? Q l 34 FEED n |3 U 1 f l i u. 2' T L CONTROLLER ls I STEAM J ll? I6 @g8 35 j v F/G.

RETTLE PRODUCT I ,f 32 12J/if AMPLIFIER 44 g 46 62 6o i' 35 RECORDER 68 p, 597 u.

CONTROLLER 56 45 67 INVENTOIL" m57 E.D.TOL|N ATTORNEYS United States Patent() 3,167,293 FRACTIGNATEN CGNTRL Ernest D. Tolin, Bartlesville, Ghia., assigner to Phillips Petroleum Company, a corporation of Delaware Filed .lnly 29, 1969, Ser. No. 46,250 4 Claims. (Cl. 23S- 151) This invention relates to the computation of internal reilux in a lfractionation column. In another aspect it relates to control systems for fractionation columns which are based on computations of internal reux.

In recent years an increasing use has been made of fan coolers for condensing overhead vapors from fractionation columns. However, this typeV oi cooler has resulted in -a r-ather serious operating prblem because it is difficult to control the exact amount of cooling provided. Such schemes as fan speed control, variable pitch fan blade control and hot vapor by-pass control have been employed in an attempt to solve this problem, but have not been entirely satisfactory. Sudden atmospheric temerature changes, such as occur during a rainstorm, for example, result in a lowering of the reux temperature. This causes an increase in the ow of liquid leaving the top tray because more of the vapor which enters this tray is condensed. This results in an increase in overhead product purity at the expense of a decreased overhead product rate.

In accordance with the present invention there is provided a novel system for computing the amount of internal reliux in a fractionation column. Internal reflux constitutes the external reflux returned to the column plus the vapor which is condensed near the top of the fractionation column by the subcooled external reflux. This computation is made from a measurement of the rate of ilow of the external reflux and a measurement of the temperature dierential between the external reflux returned to the column and a region near the top of the column. An alternating electrical signal is established which is representative of this temperature diilerential. The alterria-ting signal is applied to the primary winding of -a dilerential transformer having a movable core. The flow of external reflux is measured by Va differential pressure transducer connected across an orice in the external reilux conduit. The output of the dierential pressure transducer is applied through a non-linear linkage to adjust the position of the core in the transformer. This multiplies the input signal to the transformer by the ilow measurement to establish an output signal, representative of internal rellux, which can be employed to control the operation of the column. This control can be made automatically or by an operator from the information provided.

Accordingly, it is an object oi this invention to provide a system for computing the internal reux in a fractionation column.

Another object is to provide control systems for tractionation columns which utilize computations of the internal reflux in the column.

Gther objects, advantages and features of the invention should become apparent from the following detailed description, taken in conjunction with the accompanying drawing in which:

FIGURE l is a schematic representation of a fraction- Mice ation column having the computer and control system of this invention associated therewith.

FIGURE 2 is a schematic circuit drawing of lthe computer employed in the control system of FIGURE l.

Referring now to the drawing in detail and -to FIGURE 1 in particular, there is shown a conventional fractionation column 1G which contains a number of vapor-liquid contacting trays. A iluid mixture to be separated is introduced into column 10 through a conduit 11 at a predetermined r-ate which is maintained by a flow controller I12 that adjusts a valve 13. Steam, or other heating medium, is circulated through the lower region of column 10 through ya conduit 14 at a predetermined rate which is maintained by a iiow controller 15 that adjusts a valve 16.

Vapors `are withdrawn from the top of column 10 through `a conduit 18 which has a condenser 19 therein. Conduit 18 communicates at its second end with an accumulator 20. A portion of the condensate :in accumulator 20 is returned to column 16 as external rellux through a conduit 21. The remainder of the condensate is removed as overhead produc-t through ya conduit 22 which has a control valve 23 therein. Valve 23 is adjusted by a liquid level controller 24 to maintain a predetermined level in accumulator 26. A kettle product stream is withdrawn from the bottom of column 16 through a conduit 26 which has a control valve 27 therein. Valve 27 is adjusted by a liquid level controller 2S to maintain a predetermined liquid level in the bottom of column 10.

In order to explain the operation of the internal reilux computer and control system of this invention, an equation which is representative of the internal reflux in a fractionation column will be derived.

The material balance at the top tray of the fractionator can be expressed:

RerirVi-:Ri-iVo (1) where Re=mass ilow of liquid entering top ltray (external reux) Vir-mass llow of vapor entering top tray R=mass flow of liquid leaving top tray (internal retlux) Vo=mass flow of vapor leav-ing top tray The heat balance at the top tray can be expressed:

h=enthalpy of external reflux lz=enthalpy of internal reflux H=enthalpy of vapor streams (assumed to be equal) The enthalpy of the vapor streams entering and leaving the top tray can be expressed:

where A=heat of -vaporization of liquid on the tray.

The enthalpy of the external reflux can be expressed:

hezhi-CDAT where Cp--specitic heat of the external rellux stream AT=the difference in temperature between the top tray and external reflux Equation 3 can be substituted into Equation 2 to eliminate H and rewritten:

Equation 4 can be substituted into Equation 5 to eliminate he and rewritten:

(hd-MV1- Vo) :NRt-Re) +Recpar (6) From Equation 1 it is known:

Vi Vo=RiRe (7) Equation 7 can be substituted into Equation 6 and reduced to obtain:

Ihe computer of Ithis invention solves Equation 8. The term Re is obtained from a measurement which is representative of the rate of flow through conduit 21. This is laccomplished, for example, by the use of :a differential pressure transmitter 30 which establishes an output electrical signal representative of the differential pressure across an orice in conduit 21. It is lassumed that the temperature of the vapor leaving the top tray of column is equal to the temperature of the liquid on the top tray. The term AT is then measured 4by comparing the vapor temperature with the extern-al reflux temperature. Temperature sensitive resistance elements 31 yand 32 are connected to the input of a diiferential temperature transducer 33. The output signals from transducers 313 and 33 are 'applied to the inputs of a computer 34.

Computer 34 operates in the manner described hereinafter to establish an output signal representative of the internal reflux in the fractionation system. The output signal from computer 34 is employed to adjust the set point of a controller 3S. Controller 35, in turn, adjusts a control valve 36 in reflux conduit 21. The control system operates to maintain the computed internal reflux constant at a preselected value. If the computed internal reux should increase, valve 36 is ladjusted to reduce 'the flow through conduit 21. If the computed internal refiux should decrease, -valve 36 is adjusted to increase the ow through conduit 2.1.

Computer 34 is illustrated schematically in FIGURE 2. Temperature sensitive resistance elements 31 and 32 form two adjacent arms of 'an alternating current ybridge network 40. Variable resistors 41 and 42 form the opposite arms of the bridge. An alternating voltage source 43 is connected across first opposite terminals of the bridge, and the second opposite terminals lare connected to the input of an amplifier 44. The output of amplifier 44 is connected to the primary winding 45 of a differential transformer 46.

The electrical bridge circuit provides an output signal which is representative of the quantity of Equation 8. Variable resistors 41 and 42 are adjusted so thatY the output signal of the bridge network is representative of this quantity, the term being substantially a constant for any -given uid separation.

As previously described, the term Re of Equation 8 is representative of the flow of external reux through con- -duit 21 of FIGURE 1. A signal representative of this flow is established 'by transducer 3i) which is associated with Ian orice in conduit 21. A iirst conduit 50 extends yfrom a region upstream of -the orifice to a fluid chamber 51. 'A second conduit 52 extends from a region downstream of the oriice to ya second uid chamber 53. Chambers 51 and 53 yare separated by a flexible diaphragn 54. The position of diaphragm 54 is thus representative of the differential pressure across the orilice. This differenti-al pressure is proportional to 'the square of the rate of flow through conduit 21.

Diaphragm 54 is connected by a rod 55 to the lirst end of a lever 56 which rests on Va stationary pivot 57. The second end of lever 56 is pivotally connected yto a rod 53 which in turn is pivotally connected to the rst end of a rod 59. The second end of rod 59 is pivota'bly attached to `a stationary support 64%. A T-shaped llever 61 is pivoted at one end to a stationary support 62. A second end of lever 61 is pivotably attached to a rod 63 which is pivotably `attached to the junction between rods 58 and 59. The lthird end of lever 61 supports a core 65 which is adapted to be moved lwith respect to the coils of transformer 46. The second-ary coil 66 of transformer d6 is connected to the input of recorder-controller 35'. This coil preferably is formed of two sections which Iare connected in opposition so that there is no output signal when the core is centered.

The position of core 65`with respect to the coils of transformer 46 is Iadjusted by the position of diaphragm 54. An increase in pressure in chamber 51, for example, moves zdiaphragm S4 and rod 55 downwardly. This pivots lever 56 in a counterclockwise direction about support 57 so as to raise rod 58. This rotates rod 59 in a clockwise direction about support 66 so that lever 6l is rotated in a clockwise direction `about support 62. Such a rotation moves core 65 into the region between the transformer coils so 'as to increase the mutual inductance of the transformer. The illustrated linkage multiplies Vthe force exerted by non-linear spring 67 until this 'force is sutiicient to bal-ance the pressure differential on diaphragm 54. Spring 67 is calibrated so that movement of core 65 is a function of the square root of the differential pressure across diaphragm 54. In this manner, movement of core 65 is a direct function of the rate of flow through conduit 21.

Dierential transformer 46 multiplies the output signal of amplifier 44 .by a signal representative of the ow of external reflux so that the signal applied to controller 35 is representative of lthe internal reflux in accordance with Equation 8. The output signal of controller 35 adjusts valve 36 in FIGURE 1 so as to maintain the computed internal reflux constant at a preselected value.

In view of the foregoing description it should be evident that there is provided in accordance with this invention an improved internal reilux computer which employs electrical and mechanical elements. While the invention has been `described in conjunction with a present preferred embodiment, it should be evident that it is not limited thereto.

What is claimed is:

1. Computing apparatus comprising lirst and second temperature sensing means; means responsive to said first and second temperature sensing means toV establish a lirst alternating electrical signal representative of the difference of the temperatures off said first and second means; a transformer having a primary coil, a secondary coil, and a core movable with respect to said coils; means to establish a second signal representative of the rate of iiow of fluid through a conduit; means to apply said rst signal to the primary coil of said transformer; and means responsive to said second signal to adjust the position of said core relative to said coils to multiply said first signal by said second signal, the product being the signal induced in said secondary coil. t

2. The system of claim l wherein said means to establish said first signal comprises first yand second temperature sensitive impedance elements, ythird Yand-fourth impedance elements, a source of alternating current, and means connecting said impedance elements and said source of alternating current ina bridge network to establish a signal representative of the -diiference in temperature of said rst and second elements.

3. T he system of claim 1 wherein said means to establish said second signal `comprises an orice in the conduit, means to establish a third signal representative `of the pressure dierenti'al across said orifice, and means responsive to said third signal to establish `a signal representative of the square root of said third signal.

4. The system `of claim 1 wherein said means to establish said second signal comprises an orifice in the conduit, means forming first yand second fluid chambers, a diaphragm separating said chambers, means connecting said chambers upstream and downstream of said orice, re- 1 spectively, `and means connecting said diaphragm to said core so that movement of said diaphragm responsive to a change in the relative pressures in said chambers results in movement of said core relative to said coils.

References Cited in the file of this patent UNITED STATES PATENTS Hornfeck Apr. 20, 1948 Hornfeck Apr. 2l, 1953 OTHER REFERENCES 

1. COMPUTING APPARATUS COMPRISING FIRST AND SECOND TEMPERATURE SENSING MEANS; MEANS RESPONSIVE TO SAID FIRST AND SECOND TEMPERATURE SENSING MEANS TO ESTABLISH A FIRST ALTERNATING ELECTRICAL SIGNAL REPRESENTATIVE OF THE DIFFERENCE OF THE TEMPERATURES OF SAID FIRST AND SECOND MEANS; A TRANSFORMER HAVING A PRIMARY COIL; A SECONDARY COIL, AND A CORE MOVABLE WITH RESPECT TO SAID COILS; MEANS TO ESTABLISH A SECOND SIGNAL REPRESENTATIVE OF THE RATE OF FLOW OF FLUID THROUGH A CONDUIT; MEANS TO APPLY SAID FIRST SIGNAL TO THE PRIMARY COIL OF SAID TRANSFORMER; AND MEANS RESPONSIVE TO SAID SECOND SIGNAL TO ADJUST THE POSITION OF SAID CORE RELATIVE TO SAID COILS TO MULTIPLY SAID FIRST SIGNAL BY SAID SECOND SIGNAL, THE PRODUCT BEING THE SIGNAL INDUCED IN SAID SECONDARY COIL. 